In previous versions of Elasticsearch, closing or deleting an index during a
full snapshot would make the snapshot fail. In 5.0, the close/delete index
request will fail instead. The behavior for partial snapshots remains
unchanged: Closing or deleting an index during a partial snapshot is still
possible. The snapshot result is then marked as partial.

Thanks to several changes like doc values by default and disk-based norms,
warmers are no longer useful. As a consequence, warmers and the warmer API
have been removed: it is no longer possible to register queries that will run
before a new IndexSearcher is published.

Don’t worry if you have warmers defined on your indices, they will simply be
ignored when upgrading to 5.0.

The recent CPU usage (as a percent) has been added to the OS stats
reported under the node stats API and the cat nodes API. The breaking
change here is that there is a new object in the os object in the node
stats response. This object is called cpu and includes percent and
load_average as fields. This moves the load_average field that was
previously a top-level field in the os object to the cpu object. The
format of the load_average field has changed to an object with fields
1m, 5m, and 15m representing the one-minute, five-minute and
fifteen-minute loads respectively. If any of these fields are not present,
it indicates that the corresponding value is not available.

In the cat nodes API response, the cpu field is output by default. The
previous load field has been removed and is replaced by load_1m,
load_5m, and load_15m which represent the one-minute, five-minute
and fifteen-minute loads respectively. The field will be null if the
corresponding value is not available.

Finally, the API for org.elasticsearch.monitor.os.OsStats has
changed. The getLoadAverage method has been removed. The value for
this can now be obtained from OsStats.Cpu#getLoadAverage but it is no
longer a double and is instead an object encapsulating the one-minute,
five-minute and fifteen-minute load averages. Additionally, the recent
CPU usage can be obtained from OsStats.Cpu#getPercent.

Elasticsearch no longer allows indices to be created started with - or +, so
that the multi-index matching and expansion is not confused. It was previously
possible (but a really bad idea) to create indices starting with a hyphen or
plus sign. Any index already existing with these preceding characters will
continue to work normally.

The /_aliases API no longer supports indexRouting and index-routing, only
index_routing. It also no longer support searchRouting and search-routing,
only search_routing. These were removed because they were untested and we
prefer there to be only one (obvious) way to do things like this.

The wait_if_ongoing flag default has changed to true causing _flush calls to wait and block
if another flush operation is currently running on the same shard. In turn, if wait_if_ongoing is set to
false and another flush operation is already running the flush is skipped and the shards flush call will return
immediately without any error. In previous versions flush_not_allowed exceptions where reported for each skipped shard.